EPRoMs, or electrically programmable read only memories, are field effect devices with a floating gate structure. An EPROM floating gate is programmed by applying proper voltages to the source, drain and control gate of each cell, causing high current to flow through the source-drain path and charging the floating gate by hot electron injection. The EPROM device is erased by ultraviolet light, which requires a device package having a quartz window above the semiconductor chip. Packages of this type are expensive in comparison with the plastic packages ordinarily used for other integrated circuits. One such EPROM device is shown by U.S. Pat. No. 4,750,024, issued to John F. Schreck and assigned to Texas Instruments Inc.
EEPROMs, or electrically erasable and programmable read only memories, have been manufactured by various processes that usually require a much larger cell size than standard EPROMs and further require more complex manufacturing processes. EEPROMs can however be mounted in opaque plastic packages that reduce the packaging cost. Nevertheless, conventional EEPROMs have on the whole been more expensive on a per-bit basis.
More recently, a family of "flash" EEPROMs has been developed that allows an array of cells to be erased in bulk instead of each cell being erased individually. Because the EEPROMs may be erased in bulk, their cell size can be smaller.
To further reduce cell size and therefore the cost of manufacture, an EEPROM cell integrating an enhancement transistor with a floating-gate transistor has recently been devised. The structural characteristics of the cell and its method of manufacture have been fully disclosed in parent application Ser. No. 07/219,529, now abandoned. This application describes and claims a novel method for erasing such a cell, either on a per-bit basis or in blocks within an array of such cells.